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Digital Zoom
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Optical zoom is the
number of times the maximum focal length of a zoom lens is larger
than the minimum focal length. Consumer and prosumer cameras often
come also with a Digital Zoom, which we will discuss based on an
example of a 5 megapixel prosumer camera.
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| A. Scene shot with a 31mm lens |
B. Scene shot with a 50mm lens |
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Changing the focal length from 31mm to
50mm (50/31=1.6X optical zoom) reduces the field of view. In
image B, the sensor captures the red zone indicated in image
A. In both cases the camera will store 5 megapixel of
information into a 5 megapixel image.
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C. 1.6X Digital Zoom
Cropped and saved as lower resolution |
D. 1.6X Digital Zoom
Cropped and upsampled to full resolution |
A 1.6X Digital Zoom will only use the
information of a 1,600 x 1,200 crop and discard the rest
(2,560/1.6=1,600 and 1,920/1.6=1,200). In image C, the
camera has captured the same field of view as in image B but
only uses 2 megapixel out of the 5 megapixel resolution! If
the digital camera has the option to output 1,600 x 1,200
images, the crop will be saved as a 2 megapixel image. In
most cases, the 1,600 x 1,200 crop will be
upsampled
to the full resolution of the camera as indicated in image
D. No additional information is created in the process and
the quality of image D is clearly lower than image B.
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To Use Or Not to Use Digital Zoom
So what is the best thing to
do? If your purpose is to capture the information shown in image B,
using a lens with focal length of 50mm is of course the best option. If
you only have a 31mm lens available (or in general, if you reached the
maximum optical zoom and need to zoom in more) there are three things
you can do: |
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The recommended approach is to shoot image
A with Digital Zoom OFF and crop it later the way you want it.
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If the 5 megapixel camera has the option
to output 2 megapixel images, then shoot with 1.6X Digital Zoom
ON. The 1,600 x 1,200 crop will be saved without resampling and
2 megapixel of info is efficiently stored onto a 2 megapixel
image. You save card space compared to image A, but lose the
ability to change the way you cropped. This is recommended if
card space is critical and is equivalent to cropping in the
camera.
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It is generally not recommended to shoot
with 1.6X Digital Zoom ON and output it as a 5 megapixel image
because you are combining the disadvantages of 1. (more card
space) and 2. (lose cropping flexibility) without major
benefits*. You are saving 2 megapixel of information (crop C)
into a 5 megapixel
upsampled
image (D). Upsampling cannot create detail that was not captured
by the lens. Image B (optical zoom) has more detail than image D
(Digital Zoom).
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* If for some reason your intention is to
upsample and you are shooting in JPEG, one benefit of Digital Zoom
is that the upsampling in the camera is done before
JPEG compression.
If you shoot A, crop the 1,600 x 1,200 area, and then upsample to
2,560 x 1,920 on your computer, you will magnify the JPEG
compression artifacts and the upsampled image will look not as good
as image D. Because not all Digital Zooms are created equally, you
may want to verify the quality differences with your particular
digital camera before using Digital Zoom for this purpose.
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